The present invention relates to apparatus for implementing the method for photoelectrically determining the position of at least one focusing plane of an image inside an optical instrument, per U.S. Patent Application Ser. No. 479,525, filed June 14, 1974. In Application Ser. No. 479,525 use is made of an correlator containing imaging optics, at least one spatial frequency filter mounted at least in vicinity of an image plane of said imaging optics and a photoelectric receiver system, and light fluxes are passed through different pupil areas of the imaging optics to be modulated in common by the spatial frequency filter and split for instance geometrically or physically in correspondence with the pupil area, whereupon they are applied in sequence or simultaneously to a common or to separate photoelectric detector(s) of said photoelectric receiver system of which the output signals are processed further to control a display system and/or an adjustment device.
Apparatus to implement the method is described in Application Ser. No. 479,525. The present invention has as an object, creating further devices for implementing the method so as to obtain improved signal quality and to sense any spurious results caused in at least one of the measuring beams by any mechanical changes, and to ascertain the magnitude of such spuriousness or its effect on the measurement data and/or to eliminate such effects on the measurement data.